Ring-down spectroscopy determines the concentration of an absorbing species within an optical resonator by the measurement of a single parameter, the exponential decay rate of radiation in that resonator. Invariably, regardless of the experimental details of the particular scheme that is used, the measurement consists of detecting the radiation as it decays with an optical square law detector, amplifying the detected signal, and then processing the resulting record to determine the decay rate.
Because of the presence of fundamental broadband (white) noise in measurements, whether dominantly from the detector, the amplifier or the light itself, fluctuation and error occur in the measurement and the decay rate must be determined by one of a number of possible statistical regression (curve fitting) techniques. Often, a single measurement is insufficient for the desired precision so instead the measurement is repeated many times to produce an ensemble of records from which a more precise averaged value of the desired parameter can be obtained, provided there is no drift in the measured quantity. In the averaging process the improvement in precision is proportional to the square root of the number of records. In a practical instrument, the speed of measurement is important and the processing that achieves the desired precision with the fewest records (i.e. the shortest time) represents the preferred process.
For instruments operating in various real-world industrial environments there may often be sources of noise beyond those noted. For example, the ubiquitous sixty-Hertz line frequency from transformers, imbalanced grounds, and close proximity heavy duty machinery, can be picked-up and produce noise spectral density well in excess of the level of fundamental white noise. Careful design of the electronics package has been shown to reduce, but rarely eliminate, the effects of external noise sources. Numerical simulations have established that sixty-Hertz noise signals with a root mean square (“RMS”) amplitude comparable to that of the broadband noise, but unsynchronized with respect to the initiation of the ring-down signal, can more than quintuple the uncertainty in the measurement, resulting in a nearly twenty-five fold increase in the number of records needed to achieve the desired precision.
Therefore, it is an object of the present invention to provide a system and method that greatly reduce the effect of such noise, reducing the effect for the case sited to a mere one and a half fold increase in the number of records needed to achieve the desired precision.
Additionally, it is another object of the present invention to achieve a significant improvement in the precision of filtering any excess noise components that have a period that is greater than four times the length of a record.